DSP Blockset

Setting Block Parameters

Blocks in the DSP Blockset that have fixed-point support often allow you to specify fixed-point characteristics through block parameters. In many cases, such as with the accumulator and product output parameters, specifying these parameters enables you to simulate your target hardware more closely.

Note    The fixed-point settings discussed in this section are ignored for floating-point signals.

Most fixed-point parameters for DSP Blockset blocks appear when the Show additional parameters check box is selected, for example on the Matrix Product and FIR Decimation blocks below.

Many of the DSP Blockset blocks with fixed-point capabilities share common parameters, though each block may have a different subset of these fixed-point parameters. The following parameters are discussed in this section:

• Allow Overrides from DSP Fixed-Point Attributes Blocks Parameter
• Output Parameters
• Accumulator Parameters
• Product Output Parameters
• Intermediate Product Parameters
• Round Integer Calculations Toward Parameter
• Saturate on Integer Overflow Parameter

For a discussion of all the parameters of a specific DSP Blockset block, refer to the block's reference page in Block Reference.

Remember that the DSP Blockset does not allow mixed floating-point and fixed-point types on the input and output ports of its blocks. Therefore, the parameters discussed in this section only take effect if you feed the input port of that block with a fixed-point signal.

Allow Overrides from DSP Fixed-Point Attributes Blocks Parameter

This parameter, which is selected by default, allows fixed-point parameters to be set at the system or subsystem level. Refer to DSP Fixed-Point Attributes Block.

Output Parameters

In many cases you may specify the output data type and scaling of fixed-point DSP Blockset blocks:

• The Fixed-point output attributes parameter allows you to specify whether you want the output data type and scaling to automatically be the same as the input, or whether you'd like to specify the output word length and fraction length yourself in the dialog.
• The Output word length parameter allows you to specify the word length of the output, if you chose to do so in the Fixed-point output attributes parameter.
• The Output fraction length parameter allows you to specify the fraction length of the output, if you chose to do so in the Fixed-point output attributes parameter.

Accumulator Parameters

Fixed-point DSP Blockset blocks that must hold summation results for further calculation usually allow you to specify the data type and scaling of the accumulator. Most such blocks cast to the accumulator data type prior to summation:

Refer to the reference page of a specific block in Block Reference for details on the accumulator data type of a specific block:

• The Fixed-point accumulator attributes parameter allows you to specify whether you want the accumulator data type and scaling to automatically be the same as the output or an input, or whether you'd like to specify the accumulator word length and fraction length yourself in the dialog.
• The Accumulator word length parameter allows you to specify the word length of the accumulator, if you chose to do so in the Fixed-point accumulator attributes parameter.
• The Accumulator fraction length parameter allows you to specify the fraction length of the accumulator, if you chose to do so in the Fixed-point accumulator attributes parameter.

Product Output Parameters

Fixed-point DSP Blockset blocks that must hold multiplication results for further calculation usually allow you to specify the data type and scaling of the product output:

Refer to the reference page of a specific block in Block Reference to learn about the product output data type for a specific block. Note that for complex-complex multiplication, the multiplication result is in the accumulator data type. Refer to Multiplication Data Types for more information on complex fixed-point multiplication in the DSP Blockset:

• The Fixed-point product output attributes parameter allows you to specify whether you want the product output data type and scaling to automatically be the same as the output, an input, or the accumulator; or whether you'd like to specify the product output word length and fraction length yourself in the dialog.
• The Product output word length parameter allows you to specify the word length of the product output, if you chose to do so in the Fixed-point product output attributes parameter.
• The Product output fraction length parameter allows you to specify the fraction length of the product output, if you chose to do so in the Fixed-point product output attributes parameter.

Intermediate Product Parameters

Fixed-point DSP Blockset blocks that feed multiplication results back to the input of the multiplier usually allow you to specify the data type and scaling of the intermediate product:

Refer to the reference page of a specific block in Block Reference to learn about the intermediate product data type for a specific block:

• The Fixed-point intermediate product attributes parameter allows you to specify whether you want the intermediate product data type and scaling to automatically be the same as the input, or whether you'd like to specify the intermediate product word length and fraction length yourself in the dialog.
• The Intermediate product word length parameter allows you to specify the word length of the intermediate product, if you chose to do so in the Fixed-point intermediate product attributes parameter.
• The Intermediate product fraction length parameter allows you to specify the fraction length of the intermediate product, if you chose to do so in the Fixed-point intermediate product attributes parameter.

Round Integer Calculations Toward Parameter

Use this parameter to specify the rounding method to be used when the result of a fixed-point calculation does not map exactly to a number representable by the data type and scaling that stores the result:

• Floor, which is equivalent to truncation, rounds the result of a calculation to the closest representable number in the direction of negative infinity.
• Nearest rounds the result of a calculation to the closest representable number, with the exact midpoint rounded to the closest representable number in the direction of positive infinity.

Saturate on Integer Overflow Parameter

Use this parameter to specify the method to be used if the magnitude of a fixed-point calculation result does not fit into the range of the data type and scaling that stores the result:

• If you select this parameter, positive overflows will saturate to the largest positive number of the data type. Negative overflows will saturate to the largest negative number.
• If you do not select this parameter, overflows will wrap.

Specifying Fixed-Point Attributes

Specifying System-Level Settings